From memory, if you were to feel underneath armrest near to where it pivots from its base near the seat back (or indeed raise it as far as it goes in order to see) there should be a small metalic button. If you were to clasp/press/manipulate this then the armrest should lift further to a fully raised position. When you lower it back down you should notice a positive click as you motion the armrest back within its normal range of limitation. Repeat as needed.